1. Field of the Invention
The process of this invention deals generally with selective crystallization, and more particularly with the selective crystallization of cuprous chloride from particular solutions containing particular amounts of cupric chloride.
2. Prior Art
The separation of cuprous chloride from cupric chloride solutions possessing one or more of a number of metal impurities presents a problem, particularly in the rapidly developing hydrometallurgical copper recovery processes. As is well known, the main sources of copper today are copper sulfide ores, primarily chalcopyrite. Conventional pyrometallurgical techniques for recovering copper from its sulfide ores are objectionable due to the production of sulfur dioxide, a major air pollutant. Accordingly, hydrometallurgical developments are now being considered in the copper industry to produce pollution free processes for the recovery of copper from its sulfide ores.
Many of these hydrometallurgical processes are concerned with leaching the copper sulfide ore with ferric chloride and/or cupric chloride to form elemental sulfur prior to the recovery of the copper. The sulfur dioxide pollution problem is eliminated in these processes by converting of the sulfide sulfur directly to elemental sulfur.
One of the principal difficulties in these processes is the complete conversion of the copper in the copper sulfides to cuprous chloride, the preferred intermediate for the production of elemental copper. Generally the leaching reactions produce a mixture of cuprous chloride, cupric chloride and ferrous chloride. The prior art then reduces the cupric chloride to cuprous chloride, generally by means of elemental copper, in order to produce a solution containing only cuprous chloride and ferrous chloride, which may then be conventionally treated for the production of copper. This is necessary in that cupric chloride is not easily reduced to elemental copper in the presence of the various impurities which exist in the solutions, and also due to the fact that substantially more energy is required in order to perform this reduction. U.S. Pat. No. 3,798,026 to Milner illustrates such a process. Milner leaches his copper concentrate to produce a solution containing cuprous, cupric and ferrous chlorides, reduces the cupric chloride to cuprous chloride by means of cement copper, crystallizes a portion of the cuprous chloride from the resulting leach solution and reduces this cuprous chloride by means of hydrogen reduction to elemental copper, and treats the mother liquor from the crystallization step in order to produce cement copper, regenerate the leach reagents and remove the various impurities.
Another similar process is described in U.S. Pat. No. 3,785,944 to Atwood. This process discloses the recovery of metallic copper from chalcopyrite by leaching the chalcopyrite with ferric chloride to produce cupric chloride, reducing a portion of the cupric chloride to cuprous chloride by reacting it with fresh chalcopyrite feed, reducing the remaining cupric chloride to cuprous chloride with metallic copper, reducing the cuprous chloride to metallic copper by electrolysis and conventionally regenerating the ferric chloride leach reagent and removing the impurities.
These and other similar processes represent notable advances in the art, but possess several important drawbacks. The electrolytic recovery of copper directly from the reduced leach solution, as disclosed in Atwood, produces a relatively impure grade of copper due to the amount of impurities plated with the copper during electrolysis. Also, in order to reduce the cupric chloride to cuprous chloride it is necessary to utilize elemental copper which has already been processed. This elemental copper is oxidized to cuprous chloride by the reaction with cupric chloride. Hence, this copper must remain in the process for a relatively lengthy period of time and additional energy must be consumed in order to again convert the cuprous chloride to elemental copper.
The Milner process represents an advance in the purity of the copper produced since in this process the cuprous chloride is first crystallized from the leach solution prior to its reduction to elemental copper. However, since a substantial amount of process impurities crystallize with the cupric chloride, Milner must either remove these impurities prior to crystallization or further treat the cuprous chloride crystals in order to remove the impurities. Furthermore, Milner's method of crystallization requires that all of the cupric chloride be reduced by means of elemental copper to cuprous chloride prior to the crystallization step, and as mentioned earlier this requires a substantial energy expense from the standpoint of oxidizing elemental copper which had previously been reduced and also requires a substantially prolonged residence time before all of the copper is ultimately produced.
The process of this invention overcomes these drawbacks and presents several significant advantages. A particularly important advantage which results from the application of this process is that a substantially increased amount of cuprous chloride may be maintained in and therefore crystallized from the solution. The addition of cupric chloride increases the capacity of the solution for cuprous chloride while simultaneously minimizing the amount of iron in solution. As iron in solution presents a considerable problem during the separation of the cuprous chloride crystals from solution and the subsequent washing of the crystals, minimizing the amount of iron is highly desirable.
Another advantage results when this crystallization is carried out in the presence of one or more metal impurities commonly encountered in copper bearing ores. It has been discovered that when the cuprous chloride is crystallized from a solution containing a substantial amount of cupric chloride that the amounts of certain impurities crystallized is vastly reduced. The cupric chloride apparently inhibits the inclusion of these impurities with the cuprous chloride crystals. The resulting cuprous chloride crystals are observed to be so pure in some instances that they may be directly reduced to elemental copper without the necessity of any additional purification processing. The crystallization step of this process may therefore be carried out without the necessity of first removing these impurities, as is required in the Milner process.
Furthermore, another primary advantage is recognized from the standpoint of the amount of energy required to conduct the process. As earlier mentioned when elemental copper is employed to reduce cupric chloride to cuprous chloride prior to crystallization the elemental copper is oxidized to cuprous chloride. The initial energy required to produce this elemental copper is wasted since additional energy must be consumed to again reduce the cuprous chloride to elemental copper. The process of the present invention obviates the reduction of this cuprous chloride, thereby saving the considerable additional energy.